Early Paleozoic ridge subduction in the Chinese Altai: Insight from the abrupt change in zircon Hf isotopic compositions

Abstract

Zircons were separated from granitoids, gneisses, and sedimentary rocks of the Chinese Altai. Those with igneous characteristics yielded U-Pb ages of 280-2800 Ma, recording a long history of magmatic activity in the region. Zircon Hf isotopic compositions show an abrupt change at ~420 Ma, indicating that prior to that time the magmas came from both ancient and juvenile sources, whereas younger magmas were derived mainly from juvenile material. This may imply that the lithosphere was significantly modified in composition by a rapid addition of melt from the mantle. We suggest that this dramatic change was due to the onset of ridge subduction, which can account not only for the formation of voluminous granitoids, mafic rocks with complex compositions, and the association of adakite + high-Mg andesite + boninite + Nb-enriched basalt, but also for the coeval high-T, low-P metamorphism.Paleozoic, ridge subduction, Hf isotope, granite, Altai Zircon is ideal for Hf isotopic study, because the crystal chemistry of zircon allows entrain of large amounts of Hf, but not Lu, thus the radiogenic daughter of 176 Hf from the decay of 176 Lu is extremely low in zircon, and the 176 Hf/ 177 Hf ratios of the zircon reflect those of the precursor magma [1] . Significant advances have been made on zircon Hf isotopic systematics, for example the Laboratory of Lithospheric Evolution in the Institute of Geology and Geophysics, Chinese Academy of Sciences has contributed greatly to the technological aspects of this work.For this study, samples were collected from granitoids, gneisses, and sedimentary rocks of different age in the Chinese Altai and analyzed in the above-mentioned Laboratory of Lithospheric Evolution. Our results show an abrupt change in zircon Hf isotopic compositions at ~420 Ma, which may be result of an important regional geological event. Based on an evaluation of the regional geology, geochemical data, and metamorphic development, we suggest that the observed change may mark the onset of ridge subduction. We plan to test this model with further research.

Variable sediment flux in generation of Permian subduction-related mafic intrusions from the Yanbian region, NE China

“…1a, Sengör et al, 1993;Sun et al, 2009;Zhou and Wilde, 2013;Xiao et al, 2015;Xu et al, 2015). It comprises subduction-accretion complexes, including voluminous juvenile granitoids and their extrusive equivalents, formed during the multiple events related to subduction of the Paleo-Asian Ocean during Paleozoic.…”

Geochemistry, zircon U–Pb ages and Hf isotopes of the Paleozoic volcanic rocks in the northwestern Chinese Altai: Petrogenesis and tectonic implications

“…Recent zircon U-Pb dating revealed that early Devonian (380-415 Ma) was the most important period for magmatism in the Chinese Altai (Wang et al, 2006;Yuan et al, 2007;Tong et al, 2007;Briggs et al, 2007;Sun et al, 2009;Cai et al, 2010). During this period, a subduction-related setting was suggested by geochemical studies of mafic to intermediate volcanic rocks, granitoids and clastic rocks in the Chinese Altai (Niu et al, 1999(Niu et al, , 2006Xu et al, 2003;Zhang et al, 2004;Chen et al, 2006;Long et al, 2008aLong et al, , 2010.…”

“…In the Chinese Altai, Paleozoic granitoids and sedimentary rocks are widely exposed (Wang, 1983;Peng, 1989;BGMRX, 1993;Windley et al, 2002;Yuan et al, 2007;Long et al, 2007Long et al, , 2008aSun et al, 2008Sun et al, , 2009. Previous geochemical studies have revealed that the widespread Paleozoic granitoids display high e Hf (t) and e Nd (t) values, indicating significant addition of juvenile materials to the source of the magmas (Jahn et al, 2004;Wang et al, 2006;Yuan et al, 2007;Sun et al, 2008Sun et al, , 2009). However, geological processes for generating the voluminous granitoids and crust-mantle interaction were poorly constrained, and a well…”

Asthenosphere–lithosphere interaction triggered by a slab window during ridge subduction: Trace element and Sr–Nd–Hf–Os isotopic evidence from Late Carboniferous tholeiites in the western Junggar area (NW China)

“…In contrast, many studies have argued that Phanerozoic CAOB granitoids, which are characterized by positive ε Nd values, were generated by extensive underplating of mantlederived basaltic magmas in a post-orogenic or intra-plate extensional setting (Jahn et al, 2000;Wu et al, 2002). More recently, suggested that the CAOB contains many key features that may be related to Paleozoic ridge subduction, and a number of local case studies within the CAOB support the ridge subduction model (Geng et al, 2009;Jian et al, 2008;Sun et al, 2009;Tang et al, 2010Tang et al, , 2012a. In addition, an Early Permian (280-270 Ma) mantle plume model has also been proposed for tectonic evolution in Tarim and the CAOB (e.g., Zhang et al, 2010;Zhou et al, 2004Zhou et al, , 2009.…”